User equipment, UE, also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a wire connected telephone and/or between a user equipment and a server via a Radio Access Network, RAN, and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller, RNC, e.g. in Universal Mobile Telecommunications System, UMTS. The RNC, also sometimes termed Base Station Controller, BSC, e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project, 3GPP, Long Term Evolution, LTE, radio network nodes, or base stations, which may be referred to as eNodeBs or eNBs, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access, WCDMA, access technology. UMTS Terrestrial Radio Access Network, UTRAN, is essentially a radio access network using wideband code division multiple access for user equipment units. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
The 3GPP is responsible for the standardization of GSM, UMTS, LTE and LTE-Advanced. LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative UMTS.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the radio network node.
It may be desired, e.g. in LTE that flexibility for transmissions between a radio base station and a mobile terminal over a radio link is enabled. For this purpose, carrier bandwidths between 1.4 MHz and 20 MHz are supported, as is both Frequency Division Duplex, FDD, and Time Division Duplex, TDD, so that both paired and unpaired frequency spectrum can be used. For FDD, the downlink, i.e. the link from the base station to the mobile terminal, and uplink, i.e. the link from the mobile terminal to the base station, use different frequencies so called “paired frequency spectrum” and may hence transmit simultaneously. For TDD, uplink and downlink use the same frequency “unpaired” frequency spectrum” and cannot transmit simultaneously, as it would result in signal interference. Uplink and downlink may however share the time in a flexible way, and by allocating different amounts of time, such as the number of subframes of a radio frame, to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in uplink and downlink.
The above asymmetry also leads to a significant difference between FDD and TDD. Whereas for FDD, the same number of uplink and downlink subframes is available during a radio frame, for TDD the number of uplink and downlink subframes may be different. In LTE time is structured into radio frames of 10 ms duration, and each radio frame is further divided into 10 subframes of 1 ms each. One of many consequences of this is that in FDD, a mobile terminal can always send feedback in response to a data packet in an uplink subframe subject to a certain fixed processing delay. In other words, every downlink subframe can be associated to a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e. to each uplink subframe is associated exactly one downlink subframe. For TDD however, since the number of uplink and downlink subframes during a radio frame may be different, it is in general not possible to construct such one-to-one association. For the typical case with more downlink subframes than uplink sub-frames, it is rather so that feedback from several downlink subframes requires to be transmitted in each uplink subframe. Thus the confirmation of transmitted data, i.e. an acknowledgement (ACK)/Non-acknowledgement (NAK) may be transmitted differently in FDD and TDD.
In the following, an ACK will also be denoted as a positive acknowledgement and a NAK will also be denoted as a negative acknowledgement. Sometimes, a NAK is also abbreviated as a NACK.
In order to improve performance of transmission in both the downlink and uplink direction, LTE uses Hybrid Automatic Repeat Request, HARQ. The basic idea of HARQ, for downlink transmission, is that after receiving data in a downlink subframe the terminal attempts to decode it and then reports to the base station whether the decoding was successful by sending an acknowledgement, ACK or unsuccessful by sending a negative acknowledgement, NAK. In the latter case of an unsuccessful decoding attempt, the base station thus receives a NAK in a later uplink subframe, and can retransmit the erroneously received data.
In LTE, a radio frame of 10 ms duration is divided into ten subframes, wherein each subframe is 1 ms long. In case of TDD, a subframe is either assigned to uplink or downlink, i.e. uplink and downlink transmission cannot occur at the same time, see FIG. 1A.
The first subframe of a radio frame is always allocated to downlink transmission. The second subframe is split into three special fields, Downlink Pilot Time Slot, DwPTS, Guard Period, GP, and Uplink Pilot Time Slot, UpPTS, with a total duration of 1 ms.
UpPTS is used for uplink transmission of sounding reference signals and, if so configured, reception of a shorter random access preamble. No data or control signalling may be transmitted in UpPTS.
GP is used to create a guard period between periods of downlink and uplink subframes and may be configured to have different lengths in order to avoid interference between uplink and downlink transmissions and is typically chosen based on the supported cell radius. Thus a large cell may benefit from a longer guard period as the signal propagation time becomes longer for signals sent over longer distances.
DwPTS is used for downlink transmission much like any other downlink subframe with the difference that it has shorter duration.
Different allocations of the remaining subframes to uplink and downlink transmission are supported, both allocations with 5 ms periodicity in which the first and second half-frame have identical structure, and allocations with 10 ms periodicity for which the half-frames are organized differently. For certain configurations, the entire second half-frame is assigned to downlink transmission. Currently supported configurations use 5 ms periodicity as illustrated in FIG. 1b and 10 ms periodicity as depicted in FIG. 1c. In case of 5 ms periodicity, the ratio between downlink and uplink may e.g. be 2/3, 3/2, 4/1, etc. In case of 10 ms periodicity, the ratio between downlink and uplink may e.g. be 5/5, 7/3, 8/2, 9/1 etc.
Another developing trend in wireless communication comprises Multiple-Input and Multiple-Output, MIMO, systems. MIMO is the use of multiple antennas at both the transmitter and receiver to improve communication performance. Further MIMO offers an increase in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency (more bits per second per hertz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading). Because of these properties, MIMO has become part of modern wireless communication standards such as 3GPP LTE.
In at least some MIMO systems, the transmitting node may send multiple streams by multiple transmit antennas. The transmit streams may then go through a matrix channel comprising all paths between the transmit antennas at the transmitting node and reception antennas at the receiving node. Then, the receiving node gets the received signal vectors by the multiple reception antennas and decodes the received signal vectors into the original information.
A 4 transmitter, Tx, transmissions scheme for High-Speed Downlink Packet Access, HSDPA, is discussed within 3GPP standardization. HSDPA may also be referred to as Turbo-3G, or 3.5 G in some literature and comprise a development of the WCDMA R99 protocol. One fundamental issue with the four branch Multiple-Input and Multiple-Output, MIMO, system is how many codewords/Hybrid Automatic Repeat request (Hybrid ARQ or HARQ) processes this MIMO system should support. To reduce the signalling in uplink and downlink, it has been decided to use two HARQ processes for this system. This is because the performance of four branch MIMO with four codewords/HARQ processes is almost equal to that of two codewords/HARQ processes, while being easier to implement and define in 3GPP standard. For a two codeword-four branch MIMO, the user equipment (or the receiving node) generates up to 4 ACK/NAK information. The ACK/NAK information belonging to the same codeword/HARQ process is bundled to form a composite ACK/NAK and forwarded to the base station, or the transmitting node.
A two codeword MIMO with HARQ may comprise e.g. one ACK and one NAK belonging to the same codeword/HARQ process in the first transmission, i.e. the first transport block, TB, is ACKed while the second transport block, TB, is NACKed, as schematically illustrated in FIG. 2. The first TB belongs to a given stream and the second TB belongs to another stream, where the two streams are related to the same HARQ process. Since the composite ACK/NAK is a NAK, the base station will re-transmit the same transport blocks belonging to the same codeword. In the example above, the base station will retransmit both the first and the second transport blocks, even in the case when one of the blocks was correctly received. It may further be assumed in this exemplifying scenario that, after having retransmitted the two transport blocks, i.e. the second transmission, the first transport block, TB, is NACKed while the second transport block, TB, is ACKed. According to the composite ACK/NAK, which is a NAK, the base station again would need to re-transmit both the transport blocks. This process will be repeated, possibly in the third transmission and any further transmissions, until the receiving node, or user equipment, transmits an ACK for both the transport blocks and/or the maximum number of allowed retransmissions is reached.
Thereby is overhead signalling increased, which leads to increased signal interference, less capacity for signalling payload data, delay of signalling transmission and increased energy consumption both at the transmitting node and at the receiving node.
The above described scenario may become a problem in particular at the cell border, where the radio propagation conditions typically may be bad. Thus it may lead to that a request for handover, or a confirmation of a handover, may not be received correctly by the user equipment, which thus may advance away from the cell and lose the connection before having completed the handover procedure.